Privacy system for telephone transmission



May 6, 1930.

L. ESPENSCHIED PRIVACY SYSTEM FOR TELEPHONE TRANSMISSION Filed Dec. 4, 1928 2 Sheets-Sheet 1 #lf c ATTORNEY Y May 6, 1930. L. ESPENS'CHIED 1,

PRIVACY SYSTEM FOR TELEPHONE TRANSMISSION Filed Dec. 4, 1928 2 Sheets-Sheet 2 By-jmssed U/u verBaI ad A 4m Sabbm/ui $1 36 g 9 V a Fa Y 3111 swam gg Q, t; igfiyn 5 2nd Subbmlwl I f 3 7st suMm/ui I INVENTOR ZLZ'yaemakzed 0 V W ATTORNEY Patented May 6, 1930 NIT D STATES PATENT OFFICE LLOYD ESPENSCHIED, or KEW eABnENs, Nnw Yonx;Ass1eNon T AMnnIcAN TELE- PHONE AND TELEGRAPH COMPANY, ALGOBPORATION on NEW tom:

PRIVACY SYSTEM FOR TELEPHONE TRANSMISSION Application filed December 4, 1928. 86118111). 323,698.

This invention relates to telephone transmission systems, and more particularly to arrangements for rendering telephone conversations secret. The invention, while par- 5 ticularly applicable to radio telephone systems, is also applicable to telephone transmission over wires.

In radio telephone transmission it would be possible, if no privacy arrangements were provided, for anyone having a suitable radio receiving set to tune in and listen to a conversation. In order to overcome this difliculty, a'privacy arrangement has been proposed for telephone systems, this privacy system operating to divide the transmitted band into sub-bands which may be transposed A in their order in the frequency spectrum and,

if desired, may be inverted. Various combinations of transposed and inverted sub-bands are possible so that by frequently changing the combination employed it becomes practically impossible for an eavesdropper to listen in, even though he may know the principle upon which the secrecy of transmission is obtained and have available suitable means for re-transposing and reinverting the subbands to their normal positions.

lVith such an arrangement the widthof the total band which may be transmitted is limited, due to certain practical considerations. The frequencies which are most important from the standpoint of intelligibility are below about 2,000 cycles, the most essential frequencies being in the neighborhood of 1,000 cycles, although frequencies as low as a few .hundred cyclesare important. Frequencies somewhat above 2,000 cycles are also desirable from the standpoint of improving the quality. The limited band transmitted should therefore extend at least from a few hundred cycles to somewhat above 2,000 cycles. Now the cost of the privacy apparatus increases with the number of sub-bands into which the transmitted band is divided for the purpose of obtaining secrecy. In practice, the maximum width of a sub-band should not exceed certain limits because a sub-band of greater width in the neighborhood of or below 1,000 cycles would contain the essential speech frequencies to such an extent as'to enable an eavesdropper to tune tothis particular subband and thus obtain a suflicient degree of intelligibility for eavesdropping purposes,

. although the intelligibility could not be tolerated from the standpoint ofv ordinary commercial standards. Since the sub-bands to be interchangeable must all be of the same width, the system was designed to transmit such a number of sub-bands of the maximum width consistent with secrecy, as would include all frequencies froma few hundred up so ewhatover 2,000 cycles. To increase the n mber of sub-bands would improve the quality but at an expense of apparatus dispropor tionate to the benefit derived.

In designing a system of the type above reverred to, it was assumed that all frequencies of the total band transmitted should be transposed in position with respect to each other in order to obtain secrecy. It was, therefore, necessary to make the sub-bands of the greatest width possible in order to obtain the necessary total band width for the authorized listener. In accordance with the present invention, however, arrangements are provided whereby the frequencies outside of the sub-bands subjected to the action of the privacy device may be transmitted to the subscriber independently of the privacy apparatus. .If desired, the width of the subbands may even be decreased to a considerable extent without increasing the intelligibility. so far as the unauthorized listener is concerned. At the same time there is an increase in the quality from the standpoint of the authorized listener, by reason of the transmission without secrecy translation of frequencies lying outside the sub-bands assigned to the privacy arrangement.

The present invention takes advantage of the discovery that a band of considerable width above about 2,000 cycles may be transmitted without conveying much intelligence to the listener, particularly if, at the same time, the listener hears the frequencies below 2,000 cyclestransmitted in such scrambled form as to constitute mere noise. Therefore, by providing the privacy arrangement with by-pass circuits having suitable filters to selectively by-pass the frequencies above and/or 10c below the sub-bands transmitted through the itself permits of the transmission of a band of frequencies somewhat widerthan is usually possible with. long wave radio apparatus as heretofore designed. I

The invention may now be more fully un- 'derstood from the following description when read in connection with the accompanymg drawing in which Figure 1 is a schematic diagram illustrating the general layout of I one terminal arrangement of a two-way radio telephone circuit employing the privacy arrangement of the present invention; Fig. 2 is a schematic diagram of the pi'ivacy mechamsm for transposing and inverting the subbands while Figs. 3, 4 and 5 are curves illustrating the principles of the invention.

Fig. 1 shows the circuit arrangement of one terminal of a two-way radio telephone system, it being understood that the terminal arrangement at the distant end will be similar to that shown in Fig. 1. Referring to Fig. 1, L designates a telephone line balanced by a networ N and connected through a hybrid coil to a transmitting path TL and a receiving path R-L in such a manner that the two paths will be substantially conjugate. The transmitting path TL is connected to the radio transmitter (not shown) and the TL has associated therewith an amplifier-' detector unit conventionally indicatedat' TAD, this amplifier-detector unit being of' receiving path BL .is likewise connected to the radio receivin apparatus (not shown), these r'connections eing effected through circuit arrangements. to be described hereinafter. s v

The transmittingpath-TL includes a one way amplifier TA and a delay'circuit DT whose functions will appear later, this delay circuit being any known type of network, such, for example, as a Campbell filter, which will introduce the desired delay in transmission. Likewise, the receiving path RL in cludes an amplifier RA, a delay circuit DT. and an ampli er RA. The delay circuit DR may be similar to the delay circuit DT.

The privacy mechanism PM, which will no described in more -detail later, is common ;0 both the transmitting path TL and the receiving pathRL. In order to 'efl'ect this :ommon connection, the input side of the privacy mechanism is connected through a iybrid coil 11 to the path TL and also to a path RL' which extends to the radio receiver and comprises an extension of the receiving other, as otherwise both transmission path RL. The path BL is connectedto the midpoints of the hybrid coil so that the paths TL and RL' are substantially conjugate. In other words, signals received over the path BL will not pass through the hybrid coil to the path TL but will enter the privacy mechanism andthe balancing network PN. Likewise signals coming over the path TL will not pass through the hybrid coil to the path RL but will be transmitted in a circuit ineluding the privacy mechanism and the balancing network PN.

. The output side of the privacy mechanism has "two branches, one branch leading into the receiving path RL through the one-way amplifier RA and the other branch leading through the amplifier TA to the branch TL which extends to the radio transmitter.

The privacy mechanism-PM is of such a character that a telephone message from the line L transmitted over the path TL to the privacy mechanism and" thence over the path RL' to theradio transmitter, will be rendered unintelligible. WVith respect to speech which has been rendered unintelligible at the distant terminal, and, after being transmitted, is received over the path RL at the station shown, the same privacy mechanism PM functions (in a manner which will appear in more detail later) to restore the speech to intelligibility before transmission over the path RL to the line L. It is therefore necessary that the path TL leading to the radio trans 1 mitter, and the path RL leading from the radio receiver, should be so arranged that 'when transmission is taking place over the one, transmission cannot take place over the paths would be efli'ectively connected to the prlvacy mechanism at the same time.

In order to accomplish this result, the path the type commonly used in connection with echo suppressor circuits to operate suitable relays inresponse to applied voice currents.

The output of the amplifier-detector unit TAD is connectedto' operate four relays, 12, 13, 14,- and 15, whenever voice currents are applied to the input of the amplifier-detector- TAD over the path TL. The relay 12 normally applies a short-circuit across the path TL on'the output side of the amplifier TA as shown. Likewise the relay 13 normally applies a short-circuit to the path TL beyond the amplifier TA as indicated. Consequently, under normal conditions transmission cannot pass over the path TL to the privacy mechanism PM because of the short-circuit applied to .the relay 12, and transmission from the output of the privacy mechanism cannotpa'ss over the path TL to the radio transmitter because of the short-circuit applied to the relay 13. L

The receiving paths'R-L and RL on the contrary are normally connected to the pri-' ceiver through the privacy mechanism to the line L. But the contacts of the relays 14 and 15 are arranged to disable the paths RL and RL when said relays are operated in response to voice currents applied to TAD over the path TL.

The control of the path RL' which is exercised by the relay 15 is effected through a balancing transformer arrangement comprismg transformers 16 and 17, each having balanced windings connected as shown, so that when the contact of the relay 15 is open the two halves of the two transformers oppose each other and thereby prevent transmission over the path RL to the input of the privacy -mechanism PM. When, however, the contact of the. relay 15 is closed, as is normally the case, the two halves of the secondary winding of the transformer 17 are connected directly together so that in effect the two halves of the two transformers form parallel aiding transmission paths to permit transmission to take place from the path BL to the input of the privacy mechanism.

In a similar manner the control exercised by the relay 14 over the path BL is effected through a balanced transformer arrangement comprising transformers 18 and 19si'milar inall respects to the transformers 16 and 17 already described, these transformers being connected between the output of the amplifier RA and the delay circuit DR. \Vhen the contact of the relay 14 is epen,'the two halves of the two transformers 18 and 19 are connected in opposing relation so that transmission cannot take placefrom the output of the privacy mechanism over the path RL to tlieline L. lVhen the contact is closed, however, two parallel aiding paths are provided through the two transformers so that transmission can take place.

\Vith the arrangement above path from the radio receiver throughthe privacy mechanism to the line L is normally operative, while the path fromv the line L through the privacy mechanism to the radio transmitter is normally nonoperative, and in fact the privacy mechanism ,is electrically isolated from the paths TL and TL, This condition of aflairs' may be reversed, however, at any time when transmission enters the path TL frpm the line L. In ,order to insure, however, that ,when the distant subscriber is talking he will retain control of the circuit until he ceases talking, the path RL has associated therewith an amplifierdetector unit RAD similar to the unit TAD, already described, the output of which controls a relay 20 which when operated opens the output circuit of the amplifier-detector unit TAD associated with the transmitting described, the

and 15. Relays 14 and 15 by opening their contacts prevent transmission from the receiving path RL to the privacy mechanism PM and from the privacy mechanism PM to the receiving path RL. Relays 12 and 13 at the same time open the short-circuits across paths TL and TL, respectively, so that these paths are now in effective transmission relation with theprivacy mechanism.

The operations above described take place while the voice currents are being transmitted through the .delay circuit DT. The voice currents are then transmitted through the amplifier TA and over the remainder of the path TL to the hybrid coil 11. By means of the hybrid coil 11 the voice currents are applied to the privacy mechanism, but owing to the balanced relation of the hybrid coil circuit, are prevented from entering the receiving path BL. Upon passing through the privacy mechanism the voice currents are rendered unintelligible in a manner which will be described later, and then pass through the amplifier TA and over the path TL to the radio transmitter. Transmission to the path BL is prevented at this time by rea son of the open contact of relay 14. 3

The receiving operation at the distant ter-.

'have been rendered unintelligible by the privacy mechanism at the distant terminal are received by means of the radio receiving apparatus (not shown) and translated into waves corresponding to the Aunintelligible ,yoice waves transmitted from the distant terminal. Assuming that speech is not incoming from the line L at this time, the contacts of relays 14 and 15 will be closed so that transmission may take place through the transformers 17 and 16 to the hybrid coil 11 where the transmission divides, part of the energy flowing to the privacy mechanism and part to the artificial line PN, without, however, entering the path TL.- The rivacy mechanism functions to render inte ligible the received unintelligible ,waves in a manner to be described later. Since the path TL is short-circuit'ed by the contact of the relay 13, the received message waves are transmitted through the amplifier RA and through the transformers .19 and 18 to the delay circuit DR. Some of the energy transmitted to the delay circuit DR- also actuates the amplifier-detector RAD to energize the relay 20, thereby opening the output circuit of the amplifier-detector TAD. Consequently the relays 12, 13, 14, and are prevented m from being operated, and hence maintain the circuit in the condition shown so long'as the distant subscriber is talking. The operation of the relay takes place while the voice waves are being transmitted through the de- 15 lay circuit DR. The voice waves are then passed through the amplifier BA to the line L.

The privacy mechanism hitherto referred to may take the form illustrated schematical- 20 ly in Fig. 2. Briefly, the idea is to divide the essential part of the voice band into several equal sub-bands which, in the case assumed may each have a width of 6 cycles. These sub-bands by suitable Lmodulating 26, operations may be transposed with respect to each other in various combinations, and if desired, any or all of the sub-bands may be inverted from time to time. The o'rder of transposition and inversion of the sub-bands I0 may be changed to render it more diflicult for an eavesdropper to listen in, even when he knows the principle upon which the voice band is rendered unintelligible. In this form the now unintelligible voice band is applied to the radio apparatus and transmitted to the distant receiving station.

The same apparatus may be used for retransposing and reinverting the sub-bands of a voice band which has-been rendered um. intelligible in the manner described. Since the apparatus merely selects certain subbands and then transposes and, inverts the sub-bands, it is obvious that there will always be a combination of connections wherey I by a group of sub-bands, which have already been transposed and inverted, can be retransposed and reinverted to their normal relationship.

The apparatus shown in Fig. 2 as comprising the privacy mechanism includes an input transformer 21 for bringing the band to be operated upon to a number ofchannels 1, 2, 3. and 4. These channels include sub-band fi lters F F etc., which'may be of the band Pass type-so designed as to each pass a different sub-band of the voice band. A different sub-band is therefore applied to each of the four channels and enters the in ut circuits of each of the four modulators LL, M andM The carrier frequencies applied to these modulators dilfer from each other and are so chosen that the upper sidebands resulting from the modulating opera- I tion will all lie in the same frequency range. In the case assumed where each sub-band is to be b cycles wide, the severalparrier frequencies differ from each other by 1) cycles. Therefore, if a carrier current of frequency 0 is supplied to modulator M the carrier current applied to modulator M should have a frequency/of c+4b.- If we assume that only frequencies from f to f+4b cycles are to be transmitted, and that the band to be so transmitted is to be divided into four subbands each 6 cycles wide, it will be evident that the upper side-band in the output of each of the modulators will extend from frequency 0+7 to 0+ f+ b. The lower sidebands will not be the same, however, as that the band filter BF selects just this range' of frequencies. In this case, the selected range corresponds to the third sub-band from the bottom of the total voice band passed. In otherwords, it corresponds to-the original sub-band extending from f+2b to 'f+3b. Similarly, the range of frequencies from 0+ f to 0+f+b is also selected by the band filter BF but in this case it corresponds to the sub-band of the original voice band extending from f+b to f+2b. The corresponding bands selected by the filter BF and BF, corresponds to the lower and upper sub-bands extending from f to f+b, and from f+3b to f+4b, respectively. Consequently we have (referring to Fig. 3) the first, second, third,

and fourth sub-bands of the original voice band brought to exactly the same frequencyv level. Each channel is now transmitting the same range of frequencies but the range in any given channel corresponds to a different original sub-band frgm that of the other channels.

The output portion of the privacy mechanism comprises four corresponding channels 1', 2', 3, and 4', each connected to a common output transformer 22 Each of these channels comprisesa demodulatorsuch as DM and a band filter such as BF' Demodulators Ilt DM DM etc., are similar to the modulators M M lators 18 provided with a choice of two carrier frequencies, one higher than the band applied to it, and the other lower the lower frequency being so chosen as to shift down the applied sub-band to the position 'of a particu-' lar one of the original sub-bands but with the frequencies in their'normal order; and the higher frequency being so chosen as to shift etc., except that each of the demodu v down the applied sub-band to the same position but with the component frequencies in inverted order.

For example, let suppose the selected sub-band extending from to c+f+b cycles from channel 3, is connected to the input of the demodulator DM of channel 4'. The applied sub-band, itwill be noted, corresponds to the third sub-band of Fig. 3. m If a carrier of c-3b cycles be applied to the I band will extend from f+3b to f+4b cycles, and this sub-band may be selected by the band filter BF' to the exclusion of all other frequencies. Consequently, the final output subband of channel 4' will bea band of frequencies 1) cycles wide, corresponding to the third sub-band of Fig. 3, shifted, however, in the frequency spectrum to the position of the fourth sub-band of Fig. 3 but with the frequencies in their normal relation; that is,v the upper frequency of f+4b cycles of the output corresponding to the upper frequency f+3b of the third sub-band, and the lower frequency f+ 3b of the output corresponding to the lower frequency f+2b of the third sub-band.

Conversely, if the carrier frequency of c+2f+4b cycles is applied to the demodulator DM and the same input band, namely, the band extending from 0+ f1 cycles to -c+'f+b cycles, taken from channel No.. 3, I is applied to the demodulator, the resultant lower side-band will be the same as before, from f+3b to f+4=b cycles. In this case; however, while the final resultant sub-band is in the frequency range corresponding to the fourth sub-band of Fig. 3, and corresponds with the third sub-band of Fig. 3, the order of the frequencies in the final sub-band is inverted. In other words, the f+4b cycle frequency corresponds to the f+2b cycle frequency of the-third original sub-band and the f-l-3b frequency of the final sub-band corresponds to the f+3b frequency of the third original sub-band.

Without further description, obvious that the outputs of each of the four channels 1, 2, 3, and 4 may be variously connected to the demodulators of the channels 1", 2, 3, and 4' to effect various transpositions of the four sub-band shown in Flg. 3.

Also, by using either of the two choices of carrier frequencies for demodulators, any

desired sub-band, in addition to being shifted in frequency, may be inverted. By shifting the connections of the channels and the carrier frequencies in various combinations from time to time in such manner that corresponding combinations are simultaneously em ployed at the two terminals at the same time,

it will become practically impossible for an unauthorized listener to eavesdrop.

' It will be noted that in the arrangement- 05 above described, the final band filter, such as demodulator DM the resultant lower sideauthorized subscriber.

f+4b increased the expense of the apparatus out 40f proportion to the ultimate intelli ibility, as seen from the standpoint of t e -A less number. of channels would be impractical because even with a sub-band of maximum width, the total rangeo'f frequencies transmitted to the subscriber would be'less than the range 7 to f +46, which would not ,conform to existing standards of intelligibility. It was not possible to widen the. sub-bands beyond a given maximum without rendering it possible for an eavesdropper (who would naturally be less critical of quality) to receive a properly chosen single sub-band with sufiicient intelligibility to enable him to understand the message, particularly if he became J experienced in listening to the sort of transmission he would receive.

Now, where the character of the system is such as to permit the transmission of a wider total band than will be passed by the Privacy mechanism, the apparatus as above described,

being limited to a range from f to f+.b'

cycles, prevents the subscriber from receivin the benefit of the increased intelligibility an better quality due to the frequencies lying outside the range of the rivacy apparatus. Applicant has discovere however, that if the bandof frequencies below the lower limit of the privacy mechanism be transmitted without either inversion or shifting in the I frequency spectrum, such band of frequen cies, when added to the distorted or transit will be lated band acted upon by the privacy mechanism, does not confer any appreciable inpicked up by an unauthorized listener. The

same holds true if the band of frequencies en- This is accomplished, as shown in Fig. 1, by bridging about the privacy mechanism a bypass arrangement comprising two parallel paths X and Y. Path X includes a low pass crease in intelligibility to the output when filter LF which is divided into two halves so that a network or pad N I ma be included between the two halves of the ter to introduce a loss at the frequencies passed by the filter equivalent;; to the loss imposed on the bands freque cies passing through the privacy mechanism. In the case where the privacy mechanism passesthe band from f cycles to f+4b cycles, the filter LF may be a low-pass I m lfilter having a range from 0 to f cycles.-

pass filter arrangement HF, likewise divided "into two units Similarly, the ath Y includes the highcated by the chart of Fig. 4. Here it will be observed that the main sub-bands, which are subjected to transposition and inversion,

are the same as in Fig. 3. The band immediately below the first sub-band is, however, transmitted by the by-pass circuit X and the band immediatel above the fourth subband is transmitted by the by-pass path Y.

It has been ascertained that the frequencies which are most important from the standpoint of intelligibility are,'for the most part,

below about 2,000 cycles. The frequencies therefore only essential that above this limit ,tend to give color and character to the sounds but are of less importance from the standpoint of intelli 'bility. It is e band shiftmg and band inversion processes be applied to the It is also esirable, if possible, to make each uencies below about 2,000 cycles.

sub-band narrower than the width heretofore discussed. This is for the reason'that any eavesdropper who tunes his receivin set to pick out an individual band will fin band so narrow that it will be well below the I limit of intelligibility.

,5, the first sub-band will In order to take care of this possibility, the'freque'ncies. heretofore described as being applied to the apparatus of Figs. 1 and 2 may be altered so t at each sub-band is only 5 cycles wide and that the sub-bands cover f e the frequency range from f l t cycles, when 6' and V f are less than f'and b, and -f+4b 'is a hood of 2,000 cycles. 1 Then,

extend from f cycles to f'+b' cycles, the second from f'+b' fto f'+2b' cycles, the'third from f'+2b to f+3b' cycles and the fourth from f +36 to f+'4b cycles.

5, substantially the whole range below f +41) cycles'is-subjected to the band transposition the 4 frequency in the neighborasshown in Fig.

This gives a much narrower sub-band, and yet, as will be seen from Fig.

range is rendered unintelligible. The fre- I ,;+4b instead of f+4b. The frequencies low f may be transmitted by the lower by-pass path X by makin the cut-off of the filter LF f instead 0% f. If desired, however, these frequencies, as being relatively unimportant, may be omitted, in which case the second by-pass path 'X will be unnecessary. v

- 'The frequency assignments for the various pieces of apparatus in Fig. 2, with the alternative arrangement of sub-bands as indicated in Fig. 5, will be obvious without detailed description. For example, the four modulators, such as M M etc., will be supplied with carrier frequencies which differ rom each other by 6 cycles instead of I) cycles, and the same holds true for the carrier frequencies applied to the demodulators,

cated in Fig. 5.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely difl'erent from those illustrated, without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. In a telephone system, a transmission circuit, a privacy apparatus associated therewith, said, privacy apparatus comprising means to divide the important range of telephone frequencies from the standpoint of intelligibility into a plurality of sub-bands of such width as to be substantially unintelligible when received alone, means to variably transpose the sub-bands, and a by-pass for selectin and transmitting without transposition te ephone frequencies of small intelligibility lying without the range of the privacy apparatus.

2. In a telephone system, a transmission circuit, a rivacy apparatus associated therewith, sai privacy apparatus comprising means to divide the imp tant range of telephone frequencies from the standpoint of,

intelligibility lying without the range of the privacy apparatus.

3. In a telephone system, a transmission circuit, a privacy apparatus associated therewith, said privacy apparatus comprisin 1 means to provide the important range telephone frequencies from the standpoint of intelligibility into a plurality of sub-bands of such width as to be substantially unintelof small, intelligibility without the range of ligible When received alone, transposing the position of the sub-bands within the selected range, and transmitting telephone frequencies of small intelligibility out-sidethe selected range without transposition.

8. In a telephone system, the methodof obtaining secrecy of transmission which consists in selecting from the total telephone band the range of frequencies most essential to intelligibility, dividing the range into subbands too narrow to be substantially intelligible when received alone, transposing the position of the sub-bands within the selected range, inverting the order of the component frequencies of certain sub-bands, and transpose the sub-bands and to variably intransmitting telephone frequencies of small vert the order of the frequencies within cerintelligibility outside the selected range withtain of said sub-bands, a by-pass for transout transposition or inversion.

mitting frequencies around said privacy ap- In testimony whereof, I have signed my paratus without transposition or inversion, name to this specification this 1st day of and means to select into said by-pass tele- December, 1928. 95

phone frequencies of small intelligibility LLOYD ESPENSCHIED.

without the range of said privacy apparatus.

5. In a telephone system, a transmission circuit, a privacy apparatus associated therewith, said privacy apparatus comprising means to divide the important range'of frequencies from the standpoint of intelligibility into a plurality of sub-bands of such width to be substantially unintelligible when received alone," means to variably transpose the sub-bands, a by-pass for transmitting frequencies around said privacy apparatus without transposition, means to .select into said by-pass frequencies without the range of said privacy apparatus, and means in said by-pass to subject the frequencies so selected to substantially the same loss as is encountered by the frequencies passing through the privac apparatus.

6. In a telephone system, a transmission circuit, a privacy apparatus associated therewith, said privacy apparatus comprising means to divide the important range of frequencies from the standpoint of intelligibility into a plurality of sub-bands of such width'as to be substantially unintelligible when received alone, means to variably transpose the sub-bands and to variably invert the order of the frequencies within certain of said subbands, a by-pass for transmitting frequencies around said privacy apparatus without transposition or inversion, means to select into said by-pass frequencies without the range of said privacy apparatus, and means in said bypass to subject the frequencies so selected to 10 ligible when received alone, means to variably transpose the sub-bands, a by-pass for transmitting frequencies around said privacy apparatus without transposition, and means to select into said by-pass telephone frequencies the privacy apparatus.

4. In a telephone system, a transmission circuit, a privacy apparatus associated therewith, said privacy apparatus comprising 20 means to divide the important range of telephone frequencies from the standpoint of intelligibility into a plurality of sub-bands of such width as to be substantially unintelligible when received alone, means to variably 

